The commuter train involved in the Glenbrook disaster was travelling at an estimated speed of 37kmh when it hit the Indian Pacific, a British expert said today.

Kevin Thomas Crofts, a mechanical engineer who has worked for 20 years studying the structural integrity of trains was brought out to Australia from Derby, England to examine the aftermath of the December 2 collision.

The collision near Glenbrook in the New South Wales Blue Mountains left seven people dead and 51 injured.

Mr Crofts told the inquiry into the crash he used five factors to estimate the speed of impact: the extent of the damage, the gradient of the track, the braking performance of the interurban train, the time delay between the driver applying the brakes and them coming into effect and the reaction time of the driver.

If the interurban driver reacted 0.4 of a second after seeing the Indian Pacific on the track ahead of him, and the Indian Pacific was travelling at 6kmh Mr Crofts said the impact speed was in the range of 34-41kmh.

'The most probable actual velocity was probably around 37kmh,' he told the special commission of inquiry into the crash.

Mr Crofts said the interurban train was likely to have been travelling at a speed of 53.5kmh when the driver first spotted the Indian Pacific ahead of him.

He said the damage was intensified because the Indian Pacific's last carriage, a car carrier, and the interurban carriage were of different structures.

Very little of the energy of the crash was absorbed by the Indian Pacific car carrier.